This invention is directed to a method of producing hydrazines, and more particularly to a method of producing high purity unsymmetrical dimethylhydrazine.
UDMH is employed primarily as a fuel for liquid propellant rockets and only relatively small amounts are used industrially for the preparation of agricultural chemicals. As a rocket fuel, UDMH is usually not used neat, but in mixtures containing other hydrazines or amines. These mixtures yield liquid propellants and explosives having proper ballistic, chemical, and physical properties. Some examples are: Aerozine 50, 50% UDMH and 50% hydrazine; MAF-1, 39% UDMH, 50% diethylenetriamine, 10% Acetonitrile, and 1% water; MAF-3, 20% UDMH and 80% diethylenetriamine, and MAF-4, 60% UDMH and 40% diethylenetriamine.
The above illustrates that UDMH is a material which is of great strategic importance to the Defense Department. It is constantly being used and it is imperative that a certain stock-pile is maintained. The average annual consumption of UDMH is in the million pound range. UDMH was prepared in the past on a commercial scale by two different processes, the aqueous Raschig Process and the Nitrosamine Process.
The Raschig Process is outlined below: ##STR1##
The main problem with this process is that the UDMH has to be isolated from dilute aqueous solution (1-3%) which is a complicated energy consuming operation. UDMH cannot economically be distilled from water as long as its concentration is below approximately 15%, because there is only a minor difference in the compositions of the liquid and the gas phase. To achieve a separation, large quantities of a base such as sodium hydroxide have to be added to the liquid phase and/or the distillation has to be carried out under pressure.
The Nitrosamine Process is outlined below: ##STR2##
This process has the advantage of producing relatively highly concentrated (15-25%) solutions of UDMH in water which makes the isolation easier and less costly. The entire process in general is more cost effective than the Raschig method. A serious disadvantage of the Nitrosco Process is that the dimethylnitrosamine intermediate is known to be a strong carcinogenic material. The severity of this problem is illustrated by the fact that a large capacity commercial UDMH plant, based on this process, was shut down because of health hazard considerations.
Work described in the literature indicated that UDMH could also be prepared by a modified Raschig Process. It consists of reacting chlorine and an excess of ammonia in the gas phase to chloroamine and then introducing it into liquid dimethylamine (DMA) to form UDMH: EQU 2NH.sub.3 +Cl.sub.2 .fwdarw.NH.sub.2 Cl+NH.sub.4 Cl EQU NH.sub.2 Cl+DMA.fwdarw.UDMH+HCl
The main problem encountered in trying to scale-up this process was that in conjunction with UDMH a relatively high amount (5-20%) of the formaldehyde hydrazone of UDMH, a side product, was formed. Because of the similarity in the boiling points of the side product and UDMH, it is extremely difficult to effectively separate the two materials by distillation. Since the military specification for the rocket fuel calls for a minimum content of 98% UDMH, any material prepared in the above mentioned fashion is useless. Purification methods other than distillation were found to remove the side product; however, for large scale production they were judged not feasible for economic reasons.
The prior art, included herein by reference, and defined in "Reaction of Chloramine with Anhydrous Primary and Secondary Amines," by G. M. Omietanski, A. D. Kelmers, R. W. Shellmann, and H. H. Sisler, J.A.C.S. 78,3847 (1956) and in U.S. Pat. No. 2,806,851, do not differentiate between UDMH and its formaldehyde hydrazone contaminant. The reason for this is that these test results were analyzed by oxidation with Potassium Iodate, and this system is unable to make that distinction.